As an electrophotographic image forming apparatus, tandem color laser printers are known. For example, an image forming apparatus includes, in a body casing, a process unit including image formation units provided for each color. Each of the image formation units includes a photosensitive drum, a developing device, and a charger. The process unit is removable from the body casing in a sideways direction so that the developing device can be replaced when the process unit is pulled out. The body casing includes a conveyor belt that is disposed below the process unit and conveys a sheet, a scanner unit that is disposed above the process unit to irradiate the photosensitive drum of each image formation unit with laser light, and a fixing device disposed at the rear of the process unit (on a downstream side with respect to a sheet feeding direction). Such an image forming apparatus generally includes an exhaust duct that communicates with an exhaust outlet and an exhaust fan disposed inside the exhaust duct to remove heat generated in the body casing. The exhaust duct intake opening is, for example, between the fixing device and the process unit so that air around the fixing device and air around the lower portion of the process unit can be routed outside through the exhaust duct.
However, as the above-described configuration does not include anything to cut off heat between the process unit and the scanner unit, heat built up around the process unit travels to the scanner unit, and the temperature in the scanner unit is increased. When the temperature in the scanner unit becomes high, the housing of the scanner unit expands, and laser light may be irradiated on the photosensitive drum out of position, which may produce an adverse effect such as misalignment of colors in images.